Microelectronic devices, micromechanical devices, and other devices with microfeatures are typically formed by constructing several layers of components on a workpiece. In the case of microelectronic devices, a plurality of dies are fabricated on a single workpiece, and each die generally includes an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The dies are separated from each other and packaged to form individual microelectronic devices that can be attached to modules or installed in other products.
One aspect of fabricating and packaging such dies is forming interconnects that electrically couple conductive components located in different layers. In some applications, it may be desirable to form interconnects that extend completely through the dies or through a significant portion of the dies. Such interconnects electrically couple bond-pads or other conductive elements proximate to one side of the dies to conductive elements proximate to the other side of the dies. Through-wafer interconnects, for example, are constructed by forming deep vias on the front side and/or backside of the workpiece and in alignment with corresponding bond-pads at the front side of the workpiece. The vias are often blind vias in that they are closed at one end. The blind vias are then filled with a conductive fill material. After further processing, the workpiece is thinned to reduce the thickness of the final dies. Solder balls or other external electrical contacts are subsequently attached to the through-wafer interconnects at the backside and/or the front side of the workpiece. The solder balls or external contacts can be attached either before or after singulating the dies from the workpiece.
Conventional processes for forming external contacts on through-wafer interconnects include (a) depositing a dielectric layer on the backside of the workpiece, (b) forming a photoresist on the dielectric layer, (c) patterning and developing the photoresist, (d) etching the dielectric layer to form holes aligned with corresponding interconnects, (e) removing the photoresist from the workpiece, and (f) forming conductive external contacts in the holes in the dielectric layer. One concern with forming external contacts on the backside of a workpiece is that conventional processes are relatively expensive because patterning the photoresist requires a mask. Masks are expensive and time-consuming to construct because they require very expensive photolithography equipment to achieve the tolerances required in semiconductor devices. Accordingly, there is a need to reduce the cost of forming external contacts on workpieces with through-wafer interconnects.